Metal surfaces, especially those containing iron, nickel, chromium, cobalt, molybdenum, and alloys and combinations thereof, are prone to the accumulation of both filamentous and amorphous carbon when subjected to high temperature reactions involving carbon-containing materials, e.g., hydrocarbons and carbon monoxide. Examples of such reactions, which are of commercial importance, are the production of ethylene by cracking, the production of motor fuels from petroleum sources by conversion of heavy feedstocks, the production of vinyl chloride from dichloroethane, and the production of CO and H.sub.2 by steam reforming of hydrocarbon feed stock over a nickel-supported catalyst. Such reactions are generally accompanied by the accumulation of carbon on the surfaces of the reaction tubes in contact with the reaction medium. This accumulation of carbon in the reaction tubes causes a restricted flow of the reaction material and reduced heat transfer from the reaction tube to the reaction medium. It also causes damage to the inner surface of the tube owing to carburization. Frequent exposure to a carburization/oxidation cycle also accelerates corrosion, both of which reduce reactor tube life expectancy. The reduction in heat transfer necessitates raising the reaction tube temperature to maintain a constant gas temperature and production rate, thereby further decreasing the life expectancy of the tube.
Various methods have been employed to inhibit the accumulation of carbon. Such methods include steam pretreatment of the inner surface of the reactor tube to promote formation of a protective oxide film. Also, sulfur compounds are added to the process gases to poison active nickel sites and to scavenge free radical precursors of amorphous carbon. However, the rate of carbon accumulation can still be rapid under high severity conditions.
Other methods include the process taught in U.S. Pat. No. 4,099,990 for forming protection films on nickel, chromium or iron alloy substrates susceptible to coke formation. This process consists of first preoxidizing the substrate surface, then depositing thereon a layer of silica by thermally decomposing an alkoxysilane vapor.
Another method is disclosed in U.K. Pat. No. 1,529,441 wherein protective films are formed on a substrate of an iron, nickel, chromium, or alloy thereof. The protective film is applied by first depositing on the substrate surface a layer of another metal such as aluminum, iron, chromium or molybdenum by vaporization and then forming a protective oxide layer by treatment with steam or oxygen.
Heat-exchangers in nuclear reactors can be protected against carbon deposits by use of certain volatile silicon compounds such as dichlorodiethylsilane. See U.S. Pat. No. 3,560,336.
Yet another method is taught in U.S. Pat. No. 4,343,658 wherein a metal surface is first treated with tungsten, tantalum or a compound which will decompose to form tungsten, tantalum, or an oxide thereof, then subjected to a heat treatment at a temperature from about 600.degree. C. to about 1200.degree. C.
Although many of these methods have met with varying degrees of success, there is still a need in the art for developing alternative methods and compositions for protecting such surfaces. For example, although Al.sub.2 O.sub.3, TiO.sub.2, and SiO.sub.2 are widely used for protecting metal surfaces from carbon accumulation, they suffer from the disadvantage of spalling-off the metal surface at high temperatures, thereby exposing such surfaces to attack.